The invention relates to a continuous process for the production of thermoplastically processable polyurethane elastomers having predetermined and constant melt flow and thermal stability.
Thermoplastic polyurethane elastomers (TPU) have long been known. They are important industrially owing to the combination of high-quality mechanical properties with the known advantages of inexpensive thermoplastic processability. By using different chemical chain-extension components, it is possible to achieve a wide variety of mechanical properties. An overview of TPUs, their properties and applications is given, for example, in Kunststoffe 68 (1978), pages 819 to 825, or Kautschuk, Gummi, Kunststoffe 35 (1982), pages 568 to 584. TPUs are made up of linear polyols, mostly polyesters or polyethers, organic diisocyanates and short-chained diols (chain-lengthening agents). As with all polyurethane elastomers, they can be constructed either stepwise (prepolymer process) or by the simultaneous reaction of all the components in one step (one shot). In the case of the prepolymer process, an isocyanate-containing prepolymer is formed from the polyol and the diisocyanate and is reacted in a second step with the chain-lengthening agent. This process permits better control of the reaction and better phase separation than does the one shot process. TPUs may be produced continuously or discontinuously. The so-called band process and the extruder process are used as the most well known industrial production processes.
Thermoplastic processability represents a major technical advance over the earlier known processing methods involving casting. However, the production of large molded bodies with complex outlines or the production of molded bodies of small wall thickness causes difficulties to this day, since the high viscosity and limited flowability of the polyurethane melt prevents the mould from being filled completely. In addition, the specialized method of processing by extrusion tolerates an extremely small permissible variation in the melt viscosity. Attempts have therefore been made to achieve low viscosity of the thermoplastic by increasing the processing temperatures. This led, however, to burning or decomposition of the material, which is noticeable as blisters, shrinkage, streaks or tackiness.
The processes that reduce the melt viscosity (U.S. Pat. No. 4,071,505, DOS 24 18 075) or the gel content (U.S. Pat. No. 3,718,622) by addition of so-called chain terminators, such as, for example, mono-functional alcohols, represented an advance. A disadvantage of those processes is, on the one hand, that it is not possible in practice to establish a reproducibly defined viscosity. In fact, small amounts of such chain terminators have a considerable effect on the viscosity. Since, however, the amount of reactive impurities in the raw materials (e.g. in the polyol; different polyol quality batches are used) that are used is not constant, different viscosities, which exceed the tolerance limits of the processing, are obtained time after time according to the above specifications, with the same formulation and with a constant amount of chain terminator in each case, owing to different polymer conversions.
Furthermore, when polyurethanes are produced with an NCO/OH ratio of about 1, a particular viscosity is always obtained at least two different isocyanate concentrations. The melt viscosity at the higher isocyanate concentration (isocyanate excess) is achieved by a proportion of linear polyurethane and a proportion of crosslinked polyurethane. That melt viscosity is not suitable for the above-described difficult processing on account of the crosslinkages, some of which are not thermally stable. The melt viscosity to be strived for is that which is based only on the linear polyurethane, that melt viscosity being obtained with the lower amount of isocyanate.
In the case of the products produced by the processes known hitherto, it is not possible to differentiate between those viscosities during continuous production owing to the above-mentioned changes in the impurities in the raw materials. That leads to an unacceptable degree of uncertainty in production.
The object was, therefore, to seek an economical and operationally suitable process with which a thermoplastically processable polyurethane elastomer having defined and standardized (constant) melt flow behavior (constant viscosity) can be produced.
Surprisingly, there has now been found a continuous process for the production of thermoplastically processable polyurethane elastomers having improved properties. These elastomers, that are the reaction products of a reaction mixture comprising at least one substantially linear hydroxyl-terminated polyol having a number average molecular weight of from 600 to 5000, at least one organic diisocyanate, and a difunctional and/or trifunctional hydroxyl containing chain-lengthening agents having a molecular weight of from 62 to 500, are characterized in having a substantially constant melt flow (expressed as a substantially constant viscosity) and high thermal stability. The reaction mixture is characterized in that its overall NCO/OH ratio, adding all the reaction components (including the acid H compounds), of from 0.9:1 to 1.2:1. The process is characterized in that, in the start-up phase of the continuous production, the required added amount of organic diisocyanates for establishing the maximum viscosity of the melt of the polyurethane elastomer, preferably measured as the pressure build-up in front of a capillary having a defined temperature, is determined, the added amount of organic diisocyanates so determined is then added in a constant manner during the continuous production of the polyurethane elastomer and, by the additional and preferred simultaneous addition (preferably simultaneous with the addition of the chain lengthening agent) of relatively small amounts of from 0.3 to 6 mol. %, based on the chain-lengthening agents, of acid H compounds, preferably monofunctional acid H compounds, the viscosity of the melt of the polyurethane elastomer is adjusted to a constant value of  less than 90% of the previously determined maximum viscosity. Start-up phase means the beginning of the continuous process during which the amounts of the feeding streams are determined and adjusted.
It is possible in the process according to the invention to add up to 2 wt. % less organic diisocyanate than determined in the start-up phase, in order thus to obtain thermoplastically processable polyurethane elastomers having very particularly low melt viscosities.
Preferred polyols are polyesters, polyethers, polycarbonates or a mixture thereof.
Suitable polyether polyols may be prepared by reacting one or more alkylene oxides having from 2 to 4 carbon atoms in the alkylene radical with a starter molecule containing two active hydrogen atoms bonded therein. The following may be mentioned as examples of alkylene oxides: ethylene oxide, 1,2-propylene oxide, epichlorohydrin and 1,2- and 2,3-butylene oxide. Preference is given to the use of ethylene oxide, propylene oxide and mixtures of 1,2-propylene oxide and ethylene oxide. The alkylene oxides may be used individually, alternately in succession, or in the form of mixtures. Starter molecules include, for example: water, amino alcohols, such as N-alkyldiethanolamines, for example N-methyl-diethanolamine, and diols, such as ethylene glycol, 1,3-propylene glycol, 1,4-butanediol and 1,6-hexanediol. It is also possible to use mixtures of starter molecules. Suitable polyether polyols are also the hydroxyl-group-containing polymerization products of tetrahydrofuran.
It is also possible to use trifunctional polyether polyols in amounts of from 0 to 30 wt. %, based on the bifunctional polyether polyols.
The substantially linear polyether polyols preferably have molecular weights of from 600 to 5000. They may be used either individually or in the form of mixtures with one another.